In order to avoid defects in the fabrication of semiconductor devices, semiconductor manufacturers require high purity gases and chemicals for their production processes. Typical processing steps include using cleaning solvents for initial wafer preparation, wet etching, chemical vapor deposition, and the like. The presence of very minute amounts of impurities at any one step may result in contamination of the wafer and ultimately in the scrapping of the chip.
Two sources of impurities that result in wafer contamination include particulates and films. Particulates include any bits of material present on a wafer surface that have readily definable boundaries. As state of the art mask designs commonly have line widths in the sub-micron range, particulates may very easily interfere with the proper operation of the chip circuits. This is especially true in the case of charged particles, which may interfere with the electrical characteristics of the chip. Film contamination results when a layer of a foreign material remains on a chip after a processing step. Solvent residues and oils are common films that undesireably remain on a chip, cause contamination and ultimately reduce wafer yields. Additionally, the presence of heavy metals such as Fe, Ni, Cr, Cu, Al, Mn, Mo, Zn, and the like may contaminate a wafer as either a metallic film or as metallic particulates. As a means to increase yields, semiconductor fabrication houses (fabs) commonly require that their process gases meet particle specifications of less than 0.2 micron, and metals specifications on the order of 1 part per billion or less. By understanding the extremely sensitive nature of chip fabrication, one may appreciate why labs maintain such stringent purity standards for their wafer processing gases and chemicals. One may further anticipate that such standards will become more stringent in the future, as the geometry of semiconductor devices continues to get smaller.
Traditionally, semiconductor manufacturing facilities or fabs as they are referred to in the industry have used electronic grades of process gases which producers have supplied in cylinders. The cylinders contain gas volumes on the order of 40 litters and are installed in gas cabinets, which contain one or two process gas cylinders per station. The gas cabinets are maintained in a controlled temperature environment such that the vapor pressure of liquefied compressed gases may be controlled. In order to meet the ever increasing demands for semiconductor chips, labs have installed more gas cabinets. The increased number of gas cabinets has consequently challenged the user in regard to operational safety, as cylinder changes become more frequent and there is increased likelihood of component failure. Of course the increased operational and maintenance requirements also increase the likelihood that impurities may be introduced into process gas streams. Furthermore, such an approach to maintaining production is undesirable from an economic standpoint, as the need for identical delivery systems and components increases capital equipment, installation, and operational costs.
As an alternative to the above described method of delivering process gases, users of large volumes of liquefied compressed process gases have met flow demands by pumping liquid product from a storage vessel and vaporizing it prior to use. The advantage of this technique being that the pump enables a user to pressurize the delivery system according to process needs. While this method is straight forward for large volume, low purity users, the process becomes more complicated for large volume high purity users, such as the semiconductor industry. Experimental results have shown that chemical withdrawn from the liquid phase of a storage vessel contains substantially higher levels of metallic and oil contaminants than chemical that is withdrawn from the vapor phase of a storage vessel. When the withdrawn liquid is vaporized, the flow stream carries the impurities into the vapor stream to the point of use. Consequently, high purity users such as labs would have to rely on purifiers to remove the contaminants.
U.S. Pat. No. 5,242,468 discusses the problems in the semi-conductor industry and puts forth a proposed on-site solution for purifying chemicals for use in a semi-conductor fabrication house.
U.S. Pat. Nos. 4,579,566; 4,961,325 and 2,842,942 disclose methods and devices for maintaining pressurization of a cryogenic storage vessel. The 566' and '325 patents utilize vaporization of stored liquid to pressurize the vapor space in a cryogenic storage vessel. The '942 patent vaporizes cryogenic liquid using heat exchangers that provide ambient heat to vaporize the cryogen and heat exchange it with the liquid supply of the cryogen to maintain pressure. It is known to supply gaseous hydrogen from a liquid cryogen hydrogen supply by this method.
From the above discussion, it becomes clear that the semiconductor industry requires an improved technique to deliver ultra pure gases to their manufacturing processes using an operationally safe, cost effective bulk source and delivery system. Such a system must be able to maintain a minimum delivery pressure defined by the user.